GROWTH OF THE CORPUS CALLOSUM 47 



.cortex attains nearly its full thickness before the callosal fibers 

 are myelinated would contribute to the great difference in these 

 two curves at twenty to twenty-five days after birth. 



The cerebral cortex also has numerous radiating fibers, the 

 myelination period of which is nearly identical with those fibers 

 in the callosum (Watson, '03). 



In general, the growth of the callosal area has a tendency to 

 be longer continued than that of the total brain area, as is quite 

 evident from these two curves. This is in agreement with 

 the results found by Donaldson and Nagasaka ('18) in the fibers 

 of a spinal nerve. 



3. On looking at the data in table 74, 'The Rat' (Donaldson, 

 '15), we see that between birth and 365 days the decrease in the 

 percentage of water in the spinal cord is greater than that in 

 the brain; the spinal cord loses 17.6 per cent of water, while 

 the brain loses 10.5 per cent. 



This difference is due chiefly to the fact that the spinal cord 

 contains a greater proportion of 'myelinated fibers. The cal- 

 losum ought therefore to show a greater similarity to the spinal 

 cord than to the brain in this respect, and this conclusion is 

 supported by the observation of Koch (17); the human callosum 

 loses from 18 to 20 per cent of water. 



c. Growth of the callosal area on age 



To determine the growth of the callosal area on age in the 

 albino rat, the calculation according to the previous formula 

 was made for each callosal area, using age groups, and the 

 results are given in column E, table 3, and plotted in chart 2. 

 The data on the standard body weight in column C were taken 

 from table 74, 'The Rat' (Donaldson, '15). The growth curve 

 of the callosal area on age is very much the same as that plotted 

 on body weight, except that the rapid rise appears early. 



On examining chart 2 we notice three phases corresponding to 



the two phases in chart 1 in which the callosal areas are arranged 



according to the coresponding body weight ; that is, a first phase 



■ of rapid increase which covers the first ten days after birth, a 



THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 32, NO. 1 



